Keynote Lectures

SUMMARY

Rock mechanics play a crucial role in oil field development. Most hydrocarbon resources can only be accessed by drilling thousands of meters through rocks of highly variable consistency. In challenging areas, the drilling cost of a single well may exceed 10 million dollars. It has been estimated that 5-10% of the drilling costs in such areas are related to rock instability problems, offering a huge cost cutting potential for rock mechanical applications. Rock instability may also be a problem during production from sand and chalk reservoirs, requiring rock mechanical analyses and solutions. In some areas, especially shale gas and shale oil fields, induced fracturing of the rock is crucial for economical production. Recently, it has been shown that rock mechanics may offer a huge cost saving potential during field abandonment, as the rock itself may restore the sealing capability of the cap rock given the right conditions. On a larger scale, reservoir compaction and surface subsidence also imply rock mechanical challenges.

The role of rock mechanics has become more recognized over the years, with numerous examples of its cost saving potential. Significant progress in understanding and modeling of the behavior of the relevant rock types has been achieved over the last decades. However, rock mechanics is still considered more of a nuisance rather than a potential for many companies. Acquisition of rock mechanical data does not have a high priority in the industry, and lack of data is a major problem. There is a need for more advanced modelling tools as well. Thus, there are still many exciting challenges for rock mechanics scientists and engineers in oil field development.

SUMMARY

The open pit mine Geotechnical Engineer is responsible for creating slope designs that are expected by the owners, management, the workforce, the regulators, and the public domain to be stable for the life of the mine. Economic requirements are always implicit in the designs, but in today’s world producing slope angles that ensure the workers in the pit are protected against death or injury have become additional moral and legal requirements. These factors require that the Geotechnical Engineer has a skill set that at a minimum: encompasses regional and site specific knowledge of the geological model; understands the spatial and temporal distribution of the structural defects that are likely to affect the stability of the pit slopes, together with the properties of the soil and rock materials in which the slope will be excavated; and has the ability to transform this knowledge into a soil and rock mechanics slope design framework that is understood clearly and can be implemented readily by the mine planners and pit operations professionals. The objective of this keynote is to critically examine how well these requirements are being met and the most effective approach to satisfy best practice beyond 2019.

SUMMARY

At great depths, flat dipping tabular excavations presents unique rock engineering challenges. This presentation discusses recent developments in terms of design criteria, numerical modelling and support design to improve safety in these mining excavations.

To enhance the design criteria, methods to simulate the time-dependent stability of the fracture zone near the edges of tabular excavation layouts were recently developed. These methods now enable engineers to simulate the effect of different mining rates and face advance increment lengths. The models also accommodates energy dissipation computations in near the edges of these excavations. This allows the released energy to be used as a measure of seismic activity and addresses a number of the weaknesses in the traditional usage of energy release rate as a criterion for the design of these layouts.

Of further interest is the unique problems associated with the design of rockburst support. These designs are particularly problematic when the stopping width is very small. In shallow dipping orebodies, miners enter the stopes and the entire hanging wall needs to be supported. A support system is required that is initially stiff, but it should also be yieldable to survive the convergence in the back areas. When considering the three accepted key functions of modern rockburst support methodology namely, reinforce, retain and hold, the traditional support design for these shallow dipping orebodies does not meet all the requirements. The rock is highly fractured and fallouts occur between roofbolts during rockbursts. New methods to improve the areal characteristics of the support system is discussed in the presentation.

SUMMARY

The recent growth in shale gas extraction, geothermal energy development and storage of anthropogenic gases and fluids has led to increased human interaction with the Earth’s subsurface. A management challenge for these subsurface sites is to optimize extraction/storage approaches to yield maximum potential while minimizing risks. Fractures are one of the dominant factors that influence the success or failure of these management tasks because all subsurface activities perturb fluid pressures and stresses in rock, causing mechanical discontinuities to open, close, initiate, coalesce and/or propagate, while natural and engineered fluids can result in geochemical alterations that lead to crack growth. The goal to sustain production/isolation throughout the life-cycle of a subsurface site makes it necessary to detect and image fracture systems to monitor alterations as well as to link geophysical measurements to mechanical and hydraulic integrity of the subsurface rock.

In this presentation, the role of micro-scale properties and behavior on the detection of crack initiation, propagation and geochemical alteration is examined through three topics: (1) identification of a geophysical precursor for a system transitioning from meta-stability to unstable behavior with specific focus on crack nucleation, propagation and coalescence; (2) demonstration of acoustic emissions from geochemically-induced fractures; and (3) understanding the role of depositional layers and mineral fabric on tensile crack formation and geometry. The results from these studies advance current understanding of which microscopic properties of evolving fracture systems are most useful for predicting macroscopic behavior and the best imaging modalities to use to identify the seismic signatures of time evolving fracture properties.

SUMMARY

The dynamic process and mechanism of rock deformation and fracturing under in-situ conditions (ground stress, elevated temperature, and pore pressure, etc.) have drawn a lot of attention among the geomechanical researchers. The related researches play a very important role in the exploitations of resources and energies, especially unconventional oil and gas exploration, underground mining, and civil engineering. However, because of the invisibility of underground space, the filed measurements are only limited to some indirect observations. Therefore, the laboratory scale experiment is an indispensable studying approach, i.e., sampling, recovering the in-situ conditions in the laboratory, and then testing. Because the X-ray computed tomography (CT) technique has the following advantages: a) non-destructive detecting; b) capable of obtaining the strain field instead of the local strains. Different types of CT scanners (micro CT scanner, medical CT scanner, industrial CT scanner, synchrotron X-ray scanner) have been more and more widely used in the geomechanical investigations, such as, strain concentration zone, dynamic three dimensional fracture distributions during the fracturing process, fracture aperture measurements, joint shear behavior, etc. The previous efforts have shown: a) the computed tomography is a powerful and promising technique applying in geomechanics; b) combining some image processing techniques, such as digital image correlation, more useful information can be extracted from the CT images; c) except the direct observations from the 2-D slices or 3-D constructions, CT values can also be used for some quantitative analyses.

The in-situ CT scanning integrates CT scanning technique into the geomechanical testing, which is superior to the static CT scanning. Despite the capability of capturing the dynamic process, the previous in-situ CT scanning has the following limitations: a) the low-level energy X-ray restricted the sample size; b) because of the limited X-ray energy, the triaxial chambers usually made of low-density materials. On the one hand, such triaxial chamber cannot bear high confining pressure. On the other hand, the applied load usually reacts on the pressure cell wall, which limited the loading capacity; c) the stiffness of the loading frame generally cannot satisfy the requirement of the stiff testing machine. An appropriate tool is a prerequisite to achieving a goal successfully. Through five years of endeavor, we successfully developed an in-situ geomechanical testing facility featured by a linear accelerator X-ray source and multi-field coupling. It is the first ever geomechanical testing facility with the following features: a) 6 MeV linear accelerator X-ray source; b) fully functioning triaxial cell with high precision rotation; c) high stiffness loading frame; d) capable of simulating the coupling of the stress, temperature and seepage fields; e) capable of testing large-scale rock specimens. This brand-new apparatus transparentizes and digitalizes the fracturing process and fluid migration in the rock, thusly opening the black box of geomechanical tests. The practical applications are as follows: a) unconventional energy exploitation, such as shale gas, coalbed methane, dry hot rock and natural gas hydrate; b) new geological projects, such as underground storage of oil and gas, geological disposal of highly radioactive nuclear waste and underground CO2 storage; c) important engineering constructions of mining engineering, hydraulic engineering, transportation engineering, civil engineering, etc.

SUMMARY

Natural rock masses consist of solid and fluid materials. In rock mechanics and engineering, researches have made an effort to understand rock mass behavior under different mechanical and hydraulic conditions. After invention of powerful numerical method, FEM (Finite Element Method), it was introduced into rock mechanics to analyze coupling behavior of rock masses which contain water. Derivation of basic equations came from consolidation theory in soil mechanics.

Coupling analyses at first have been developed in continuous media and then extended to discontinuous jointed rock masses. Major contribution to solve coupled problems came from DECOVALEX project which deal with nuclear waste repository in deep geological formation.

Couple between solid rock and water was extended to broader area. Fully coupled thermal-hydro-mechanical analysis with multiple fractures becomes common in research area. In addition, chemical effect is under consideration. It means that very complex interaction between solid rock (stress or strain), water, air, heat, chemical reaction is coupled with time. Theoretical and experimental researches are actively performed worldwide.

Numerical methods to handle the coupled problems keep on evolving. In addition to FEM (mainly continuous), discrete methods such as DEM, DDA, MM etc. have been utilized to interpret complex natural discontinuous rock mass behavior.

Recent new development for fluid modeling is so called particle methods (MPS, SPH etc.). They are used to explore the physical and mechanical effects of water on rock mass. Interaction analyses between solid (use discrete models) and water particle reveal and visualize what is happening inside and outside of rock masses.

Various types of simple to complex coupled analyses will be summarized and some numerical examples are shown in the lecture.

SUMMARY

Tunneling through some types of soft rock has called the attention of engineers and geologists alike. The behavior of those rock masses is such that the most usual rock mass classification systems (e.g. Q, RMR, GSI) frequently fail to provide reliable predictions. Attempts to develop systems tailored to soft rock masses have not been successful either, when compared to the success of traditional systems for hard rocks. The ductility of the intact material tends to hinder the generation of discontinuities. Thus, massive character is frequently found. This characteristic also creates the ability of the material to redistribute stresses by yielding without developing much damage due to tunneling. Guidelines of maximum depth of tunnels in hard rock before rock burst starts to occur based on the ratio between either vertical or tangential stresses and the uniaxial compressive strength usually do not apply to the prediction of the limit depth before squeezing takes place in soft rocks. The so-called competence factors to predict squeezing in soft rock are usually higher than similar ratios for the prediction of the onset of rock burst in hard rock. This may be due to the fact that strength scale effect is usually much less pronounced in soft rocks. In other words, the real local ratio at the tunnel excavation perimeter between stress and strength tends to be lower in soft rocks when laboratory test results are taken. Another important characteristic of soft rocks which may affect their excavation by mechanical methods is that the selection of equipment should be based on strength values obtained from tests conducted at higher loading ratios. The cutting tools apply loads which increase at much higher rates than those adopted for usual laboratory tests. Most hard rocks present negligible strength increase (less than 20%) when the loading rate is increased by two or three orders or magnitude. In some soft rocks this increase may be of the order of 300%. Methods to estimate the excavation productivity based on conventional strength tests may overestimate the results if the tests are not conducted at appropriate loading rates.

In addition to interpretation of information from the literature, result of an extensive campaign of laboratory tests on a volcanoclastic breccia from Costa Rica will be presented as well as the general data on the performance of the construction of a tunnel excavated through that rock for the Cachí Hydroelectric Powerplant.

SUMMARY

Performance monitoring is a fundamental component of the observational design approach when applied to tunnels. The objective is to find if the design predictions on the ground behavior and system response, including the stabilization and reinforcement measures adopted, and the final lining installed are within the expected limits. The need is to continuously update the geological and geotechnical model during face advance, in order to carry out the required back analysis and understand, nearly real time, if the overall tunnel performance is as desired and, where needed, take appropriate actions on the excavation and construction method adopted.

With the above in mind, this lecture is to give first an overview of the “conventional” monitoring equipment used for displacement measurements in and around the tunnel and on the ground surface (where needed), including the instrumentation that is installed for determining the stresses in the support/reinforcement components. Equipment that is more “advanced” and methods that concur to obtain these same objectives are described, including (e.g.) robotic total station, 3D laser scanner, interferometric synthetic aperture radar, Mems sensors of different types, optical fiber sensors, including data transmissions systems and processing.

Attention is dedicated in the lecture to the evaluation of the monitoring data, including numerical methods for back analysis and assessment of the ground behavior and system response. Emphasis is placed on the need to use such methods so that the calculations performed lead to realistic results, which can be carefully checked and reflect the in situ behavior and the tunnel response realistically. It is pointed out that the ability to do so has been enhanced through the years given that the development of new monitoring tools mentioned above has been accompanied by an increased ability in acquisition and visualization of the rock mass characteristics.

Three case studies are presented: (1) A twin-tunnel (each tube is 15.1 m diameter), excavated with an EPB TBM in difficult ground (clay marl). (2) A twin tunnel (each tube is 15.6 m diameter) excavated with an EPB TBM in claystone and highly clayish sandstone in proximity of a deep-seated landslide. (3) A single tube tunnel (13.6 m diameter), excavated through a para gneiss-mica schist rock mass with a Single Shield TBM under two old tunnels excavated by conventional methods in the Sixties. The three cases are discussed with reference to the monitoring systems adopted of either “conventional” or “advanced” type, and in view of the technical problems met during excavation and in order to put them into service and allow for long-term performance monitoring.